SIP is a signalling communications protocol widely used for controlling multimedia communication sessions such as voice and videos over Internet Protocol (IP) networks.
In the ISO/OSI model, SIP is an “Application” layer protocol which is based on Internet Protocol (IP) and User Datagram Protocol (UDP), or other transport protocols, and is used to create, modify, and terminate sessions with one or more participants. These sessions include Over the Internet phone calls, multimedia distribution, and multimedia conferences. SIP “Calls” used to create sessions carry session descriptions that allow participants to agree on a set of compatible media.
SIP makes use of proxy servers to allow routing of requests to the current user positions, user authentication, user authorization for certain services, implementation of service provider-specific call routing policies and provision of services to their users.
SIP also manages the user registration function, which is handled by the proxy servers, so as to have always a real-time updated control of the users in a given area.